Tun Dr Mahathir Mohamad and his wife Tun Dr Siti Hasmah Mohd Ali joined the participants at Bersih 4 at Dataran Merdeka in a surprise visit.

Bersih 2.0 chief Maria Chin Abdullah has insisted that its two-day Bersih 4 rally will proceed as planned on Aug 29 despite it being declared illegal by the authorities.

The High Court, however, had declared Bersih as a lawful society in 2012.

The Bar Council's Human Rights Committee also said there was no such thing as an illegal rally any more.

Its co-chair Andrew Khoo said: "The courts have taken a dim view of taking action against a person exercising his or her constitutional right to peaceful assembly."

"However, with the new Section 124B of the Penal Code, attending a rally could be deemed an activity detrimental to parliamentary democracy."

"This is untested legislation and the legal environment today is different from that in 2012 (when Bersih 3 took place)," he added.

Lawyer Amer Hamzah Arshad also said each person had the right to assemble peacefully.

He said the Court of Appeal decision in PKR lawmaker Nik Nazmi Nik Ahmad's case over the Black 505 rally was still applicable.

On April 25 last year, the Court of Appeal acquitted Nik Nazmi, the Seri Setia assemblyman, ruling that his prosecution under Section 9(5) of the Peaceful Assembly Act (PAA) was null and void.

The court held that Section 9(5) - which criminalises the failure to give a 10-day notice before a gathering - was unconstitutional because it criminalised the fundamental right to gather.

Amer Hamzah urged the police to concentrate instead on those who may try to sabotage the assembly or create trouble.

"These groups should be stopped because they may want to subvert the objective of the rally."

Malaysian Bar president Steven Thiru criticised the police decision to disallow Bersih 4, stating that the right to assemble peacefully was provided for in Article 10(1)(b) of the Federal Constitution.

He said Section 27 of the Police Act 1967 required a police permit for assemblies but that provision was repealed in 2012.

Steven pointed out that the PAA had no requirement for a police permit for an assembly.

The crowds at the Bersih 4 gathering points in Bangsar, Central Market, Brickfields, Masjid Negara, Sogo and Menara Maybank are marching by the hundreds and thousands towards Dataran Merdeka.

However, a police blockade has stopped the crowd from reaching the square.

The group of assembled supporters from the various checkpoints has made base at the intersection of Jalan Tunku Abdul Rahman (TAR) and Jalan Pudu, close to the blockade, bringing traffic in the area to a near standstill.

The rally officially started at 2pm in designated locations throughout the city.

Participants were seen chanting, waving flags and banners, singing NegaraKu and blasting air horns.

Notably absent are PAS supporters who came out in full force during previous Bersih rallies.

The overnight rally will continue until 11.59pm on Sunday.